Radiation-curable composition

ABSTRACT

Compositions suitable for use as base coats and varnishes, preferably for use in combination with electrostatic of set inks printed onto plastics substrates, particularly polycarbonate compact discs. They contain a photopolymerisable material, a photoinitiator, an inorganic transfer agent and heavy metal salt, preferably a metallic carboxylate. They have a good shelf life and good adhesion after hardening with ultraviolet light.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority of Great Britain Application No.PCT/GB01/01356 filed Mar. 27, 2001 which claims the priority of GreatBritain Application No. 0007627.3 filed Mar. 30, 2000, the completedisclosure of which is hereby incorporated by reference.

This invention concerns a radiation-curable composition. In particular,this invention concerns a radiation-curable composition that can be usedas a varnish or base coat such as, for example, as a coated layerpossessing good adhesion to electrostatic offset inks.

In electrostatic offset printing an image is formed by directing one ormore pigmented inks or toners on to a selectively charged drum, whichare then transferred, either directly or via an intermediate member, onto a substrate. Electrostatic offset printers are made and sold, forexample, by Indigo NV and by Xeikon SA.

Inks used in offset printing have difficulty adhering to certainsubstrates such as plastics. This can be overcome by modifying thesurface of the substrate. The surface can be modified by the applicationof a preliminary base coat comprising, for example, a solvent-basedresin such as, for example, a polyamide, a butadiene resin or anethylene-acrylic acid resin (see JP 10076744). When a solvent-basedresin is used, the solvent needs to be evaporated before printing, whichcan be slow, hazardous and offensive, and is environmentally unfriendly.

WO 98/49604 discloses a copolymer receptor layer which is applied byextrusion to a polyvinyl chloride backing to improve the abrasionresistance of a printed image. The copolymer receptor layer can also beused to improve print quality on a polycarbonate backing layer. In bothof these applications, we are told that the copolymer receptor layeradheres to the polyvinyl chloride backing layer in the absence of UVradiation.

One aim of the present invention is to provide a composition for a basecoat that can be used on difficult substrates, such as plasticsubstrates, that are printed using, for example, electrostatic offsetprinting. A further aim of the present invention is for the viscosity ofthe composition to remain stable on storage. A further aim of thepresent invention is for the composition not to be capable ofdetrimentally attacking the surface of a compact disc. A further aim ofthe present invention is for the composition to exhibit a good bond withelectrostatic offset inks. A further aim of the present invention is toprovide a base coat that can be applied by screen process printing or byany other suitable coating or printing process.

In accordance with the present invention there is provided a compositionfor use as a base coat or varnish, the composition comprising:

-   -   (i) one or more ethylenically unsaturated monomers, oligomers or        prepolymers that are capable of being photopolymerized;    -   (ii) a photoinitiator that is capable of initiating the        polymerization of the ethylenically unsaturated monomers (i)        when irradiated with ultra-violet light;    -   (iii) an inorganic transfer agent; and    -   (iv) a heavy metal salt.

In accordance with the present invention there is also provided a methodfor printing a substrate, the method comprising the step of applying abase coat to the substrate before printing, the base coat comprising thecomposition defined above.

There is also provided a method for varnishing a printed substrate, themethod comprising the step of applying a varnish to a printed substrate,the varnish comprising the composition defined above.

In accordance with the present invention there is also provided use ofthe composition defined above as a base coat or varnish for printing.

The composition of the present invention does not contain a volatilesolvent which must be evaporated in use, it has a good shelf life, andit does not contain aggressive amine compounds. The composition may beapplied to, for example, a polycarbonate compact disc by variousprinting or coating methods, and, after hardening by ultra-violetirradiation, it gives a tough layer to which electrostatic offset inkadheres well. Another application for the composition is as a varnishapplied over a previously printed layer of electrostatic offset ink, inwhich case the varnish adheres well to the previously printed ink andpossesses good clarity and resistance to abrasion.

The base coat or varnish may be applied to a substrate using, forexample, screen process printing, flexography, curtain coating or rollercoating.

In use, the base coat or varnish is applied to a substrate and thenirradiated using ultra-violet light, typically of wavelength between 250and 450 nanometres. Preferred substrates are plastics articles, such aslabels, packaging films, graphic overlay film, membrane switch panels,and especially uncoated and spin-coated polycarbonate compact discs orDVD's.

The ethylenically unsaturated monomer, oligomer or prepolymer (i) ispreferably an ester of acrylic or methacrylic acid, such as, forexample, octyl acrylate, decyl acrylate, lauryl acrylate, phenoxyethylacrylate, hexanediol diacrylate, tri(propylene glycol) triacrylate,trimethylolpropane triacrylate, tetrahydrofurfuryl acrylate,polyethylene glycol diacrylate, hydroxyethyl methacrylate; or anN-vinylamide such as, for example, N-vinylcaprolactam orN-vinylformamide. Other useful monomers are acryloylmorpholine, theacrylate esters of the ethoxylated or propoxylated derivatives of di-,tri- or tetrahydric aliphatic alcohols, and the acrylate or methacrylateesters of epoxy-, urethane, melamine or polyester resins or theirethoxylated or propoxylated derivatives. Mixtures of monomers may beused. The monomer is preferably present in an amount from 10 to 80%,more preferably 23 to 50%, by weight of the composition.

The photoinitiator (ii) is either of the type known to produce freeradicals when irradiated with ultra-violet light, or of the “cationic”type which yields a strong protonic acid when irradiated.Photoinitiators of the former type are, for example, benzophenone,1-hydroxycyclohexyl phenyl ketone, isopropylthioxanthone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzildimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphineoxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, or a mixturethereof. Photoinitiators of this class are commercially available, forexample, under the trade names Irgacure (from Ciba), Genocure (from RahnAG), Darocure (from Ciba), Lucerin (from BASF). Photoinitiators of thecationic type are, for example, sulphonium or iodonium salts such astriphenylsulphonium hexafluoroantimonate or diphenyliodoniumhexafluorophosphate. Colourless or nearly colourless materials arepreferred. Yellow coloured photoinitiators may also be preferred. Thephotoinitiator is preferably present in a total amount from 0.5 to 20%,more preferably from 2.5 to 8%, by weight of the composition.

The inorganic transfer agent (iii) is preferably a silica or a modifiedsilica or mixtures thereof. Suitable silicas are Aerosil 200 (availablefrom Degussa) and Cab-o-sil TS610 (available from Cabot). We have foundthat a suitable inorganic transfer agent has the combined advantages ofgiving a desirable “structure” to the compositions, i.e. resistance toflow under gravity without excessive viscosity at higher shear, andrendering the final cured base coat particularly receptive toelectrostatic offset ink. The inorganic transfer agent (iii) ispreferably present in an amount from 0.5 to 50%, more preferably from 2to 45%, by weight of the composition.

The heavy metal salt (iv) is preferably a metallic carboxylate, althougha naphthenate, acetate or carbonate or any anion imparting solubility isalso effective. The metallic carboxylate is preferably the salt of analiphatic or alicyclic carboxylic acid having between 6 and 20 carbonatoms. The heavy metal cation is preferably cobalt, zirconium,manganese, cerium, strontium or barium. Examples of heavy metalcarboxylates are cobalt 2-ethylhexanoate, zirconium iso-octanoate andmanganese neodecanoate. Such compounds are known as paint driers, andare frequently available commercially under trade names, such as“Durham” from Elementis Specialties and “Manosec” from Rhone Poulenc.The use of heavy metal salts, for example, zirconium salts such aszirconium acetate or ammonium zirconium carbonate, to improve thetoughness of polymer layers, is known in the art, for example, in screenstencils (see EP 0 909 991), but the use of heavy metal driers asassistants for the adhesion of electrostatic offset ink to a basecoatcontaining them is novel and unexpected. As is known in the art, suchmetal salts may be classified by the proportion by weight of the heavymetal salt (iv) in order to give a calculated proportion of heavy metalin the total composition between 0.1 and 6%, preferably between 0.8 and4% by weight.

Optionally, other additives of known types may also be present such as,for example, passive resins (i.e. resins that are not cured byultra-violet light) to stabilise the viscosity of the composition andimprove the adhesion or other physical properties of the cured layer;dyes or pigments, especially white pigment to form an opaque backgroundfor the electrostatic offset print; stabilisers to prevent deteriorationof the composition to heat or on storage; flow and slip aids; biocides;identifying tracers, defoamers; and surfactants.

The components of the compositions are selected so that they arecompatible with each other and provide a mixture stable on storage, withthe required rheological and other properties. In particular, thecompositions are preferably formulated to give a Theological profilesuitable for application printing, especially by screen-printing. Forthis purpose their gel strength, as measured as the viscosity at 25° C.on a Brookfield RVDT digital viscometer at 5 r.p.m using a No. 6spindle, lies between 40 and 1000 but preferably lies between 100 and500 poise, and their viscosity under higher shear, as measured on theRotothinner (supplied by Sheen Instruments), lies between 5 and 65 poisebut preferably lies between 20 and 50 poise.

The substrate may be foil, paper or board, or a plastics material suchas, for example, polycarbonate, or poly(vinyl chloride). Preferred ispolycarbonate such as is used in the production of compact discs. Thesubstrate may be printed with the composition using a screen stencil orby any other appropriate printing or coating method, and then hardenedby exposure to ultra-violet light. The light may be directed directly onto the printed substrate or via a ‘cold’ mirror system or similar coldcure lamp system, using commercially available equipment, for example,from Svecia or Kammann. A typical exposure is up to 100 impressions perminute with two mercury lamps having 240 watts/centimeter power on aKamman K15 printer. After curing, the substrate bearing the base coat isespecially suitable for receiving electrostatic offset ink.

In an alternative embodiment of the invention, the substrate bears apreviously printed image, preferably an image produced using anelectrostatic offset ink. The composition is then applied to thesubstrate as a varnish using a screen stencil or other appropriateprinting or coating method and hardened as above. The result is a toughresistant layer covering the image and having good adhesion to it.

The invention will now be described with reference to the followingExamples, in which all parts are by weight:

EXAMPLE 1

The following components were mixed at ambient temperature in astainless steel pot using a Torrance stirrer until homogeneous:

Tri(propyleneglycol) diacrylate 38.7 parts Fluorstab UV-5 (stabiliserfrom Kromachem Ltd)   1 part Adhesion resin LTH (from Huels AG) 22.8parts Ebecryl 160 (acrylate monomer from UCB SA) 13.0 parts

The following components were added successively, each time withstirring until dissolved:

Durham Zirconium 18 (zirconium carboxylate from Harcros) 16.5 partsIrgacure 184 (photoinitiator from Ciba)   4 parts

The following component was added and the stirring was continued untilno nibs were detected on a Hegman gauge. During this stage thetemperature of the mixture was allowed to rise to 40 C.

Aerosil 200 (silica from Degussa AG) 4 parts

The product was finally filtered through muslin. Its viscosity at 25° C.was 500 poise measured on a Brookfield RVDT digital viscometer at 5r.p.m using a No. 6 spindle and 41 poise measured on a Rotothinner. Thecomposition was useful as a clear primer for printing on a polycarbonatecompact disc.

EXAMPLE 2

The following components were mixed as described in Example 1:

Tripropylene diacrylate 35.8 parts Ebecryl 160 13.0 parts Adhesion ResinLTH 23.2 parts Irgacure 184  4.0 parts

Co12 Hex-Cem (cobalt carboxylate from OMG Kokkola 20.0 parts ChemicalsOY) Aerosil 200  4.0 parts

The viscosity of the product was 100 poise on the Brookfield viscometerand 35 poise on the Rotothinner, in each case measure as in Example 1.

EXAMPLE 3

The following components were mixed as described in Example 1 andstirred off-gauge:

Highlink OG 103-53 75.7 parts Fluorstab UV-5  0.5 parts Durham Zirconium18 16.5 parts Irgacure 184  3.3 parts Aerosil 200  4.0 parts(Note: Highlink OG 103-53 from Clariant is a commercial mixture of anacrylate ester and a modified silica. Therefore, it contains bothcomponents (i) and (iii).)

The viscosity of the product was 10 poise on the Brookfield viscometerand 6 poise on the Rotothinner, in each case measured as in Example 1.

EXAMPLE 4

The following components were mixed as described in Example 1 andstirred off-gauge:

Tripropyleneglycol diacrylate 14.7 parts Fluorstab UV-5   1 partAdhesion resin LTH 12.3 parts Ebecryl 160 12.5 parts N-vinylcaprolactam  10 parts Durham Zirconium 18 16.5 parts Irgacure 184   4 parts LucerinTPO (photoinitiator from BASF)   4 parts

When all these components had dissolved, the following components wereadded and successively stirred until no nibs were visible on a Hegmangauge:

Finntitan RDDI (white pigment from Kemira OY) 22 parts Aerosil 200  3parts

The viscosity of the product, measured as above, was 310 poise on aBrookfield viscometer and 30 poise on a Rotothinner. When printed as aprimer on a polycarbonate substrate and exposed to ultra-violet light,it formed an opaque white base-coat which was suitable to receive acoloured image printed by the electrostatic offset process.

EXAMPLE 5

The following were mixed as described in Example 1:

Tris(hydroxymethyl)propane triacrylate 13.5 parts Tri(propyleneglycol)triacrylate 19.6 parts Fluorstab UV-5   1 part Adhesion resin LTH  3.9parts Actilane 320TP20 (epoxy acrylate from Akcros)   45 parts DurhamZirconium 18   5 parts Irgacure 184   4 parts Benzophenone  2.5 partsAerosil 200   4 parts

The mixture was stirred until no nibs were visible on the Hegman gauge,then the following components were added and stirred for 30 minutes:

Modaflow (flow aid from Monsanto)   1 part Tegoglide 410 (slip aid fromTh. Goldschmidt) 0.5 part

The viscosity of the product, measured as above, was 500 poise on aBrookfield viscometer and 42 poise on a Rotothinner. The product wasapplied as a varnish over an image printed on a polycarbonate substrateby an electrostatic offset printer and, when exposed to ultra-violetlight, gave a clear resistant layer.

EXAMPLE 6

The following components were weighed into a pot and stirred untilhomogenous. The temperature was not allowed to reach 40° C.

Highlink OG 103-53 44.5 Genorad 16 (Lambson/Rahn) 0.6 Ebecryl 220 (UBC)12.6 Glycerine (Monarch) 0.2

The following photoinitiators were added and stirring continued for 10minutes until a homogenous mixture produced:

Darocure 1173 (Ciba) 2.0 Irgacure 184 2.4 Lucirin TPO 2.8

The following were added while stirring until no nibs were visible on aHegman Gauge:

Finntitan RDDI (white pigment) 16.8 Aerosil 200 4.3

The product produced was a white ink having a viscosity at 25° C. of 23poise when measured on the Rotothinner, and 140 poise on the BrookfieldRV6/5.

1. A composition for use as a base coat or varnish, having a viscosityof 25° C. on a Brookfield RVDT digital viscometer at 5 r.p.m. using aNo. 6 spindle of between 40 and 1000 poise, and a viscosity under highershear, as measured on the Rotothinner (supplied by Sheen Instruments)between 6 and 65 poise, the composition comprising: (i) from 10 to 80%by weight of the composition of one or more ethylenically unsaturatedmonomers, oligomers or prepolymers that are capable of beingphotopolymerized; (ii) from 0.5 to 20% by weight of the composition of aphotoinitiator that is capable of initiating the polymerization of theethylenically unsaturated monomers (i) when irradiated with ultra-violetlight; (iii) from 0.5 to 50% by weight of the composition of aninorganic transfer agent; and (iv) between 0.1 and 6% by weight of thecomposition of a heavy metal salt.
 2. The composition claimed in claim1, wherein the monomer, oligomer or prepolymer is an ester of acrylic ormethacrylic acid, or a N-vinylamide, or a mixture thereof.
 3. Thecomposition claimed in claim 2, wherein the ester is phenoxyethylacrylate, hexanediol diacrylate, tri(propylene glycol) diacrylate,trimethylolpropane triacrylate, polyethylene glycol diacrylate,hydroxyethyl methacrylate, trimethylolpropane triacrylate,tetrahydrofurfuryl acrylate, lauryl acrylate, octyl acrylate, decylacrylate, acrylates of polyurethane, polyester, polyether, melamine orepoxy resins, ethoxylated or propoxylated derivatives of any of theaforementioned acrylates or methacrylates or a mixture thereof.
 4. Thecomposition claimed in claim 2, wherein the N-vinylamide isN-vinylcaprolactam or N-vinylformamide.
 5. The composition claimed inclaim 1 wherein the monomer, oligomer or prepolymer (i) is present in anamount from 23 to 50, by weight of the composition.
 6. The compositionclaimed in claim 1, wherein the photoinitiator (ii) is selected frombenzophonone, 1-hydroxycyclohexyl phenyl ketone, isopropylthioxanthone,2-hydroxy-2 methyl-1-phenylpropan-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzildimethylketal, bis(2,6-dimethlybenzoyl)-2,4,4-trimethylpentylphosphineoxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, sulphonium oriodonium salts such as triphenylsulphonium hexafluoroantimonate ordiphenyuliodonium hexafluorophosphate; or a mixture thereof.
 7. Thecomposition claimed in claim 1, wherein the photoinitiator (ii) iscolourless or nearly colourless.
 8. The composition claimed in claim 1,wherein the photoinitiator is presenting a total amount from 2.5 to 8%,by weight of the composition.
 9. The composition claimed in claim 1,wherein the inorganic transfer agent (iii) is a silica or a modifiedsilica or mixtures thereof.
 10. The composition claimed in claim 1,wherein the inorganic transfer agent (iii) is presenting an amount from2 to 35%, by weight of the composition.
 11. The composition claimed inclaim 1, wherein the heavy metal salt (iv) is a metallic carboxylate,naphthenate, acetate or carbonate.
 12. The composition claimed in claim11, wherein the metallic carboxylate is an aliphatic or alicycliccarboxylic acid having between 6 and 20 carbon atoms.
 13. Thecomposition claimed in claim 11, wherein the heavy metal salt includes acation selected from cobalt, zirconium, manganese, cerium, strontium orbarium.
 14. The composition as claimed in claim 1, wherein theproportion of the heavy metal in the heavy metal salt (iv) is between0.8 and 4% by weight of the composition.
 15. The composition as claimedin claim 1, wherein the composition has a viscosity at 25° C. on aBrookfield RVDT digital viscometer at 5 r.p.m using a No. 6 spindle ofbetween 100 and 500 poise; and a viscosity under higher shear, asmeasured on the Rotothinner (supplied by Sheen Instruments), between 30and 50 poise.
 16. The composition as claimed in claim 1, wherein thecomposition includes a pigment, preferably a white pigment.
 17. Thecomposition claimed in claim 1, wherein the composition is used inscreen process printing, flexography, curtain coating or roller coating.18. A method for printing a substrate, the method comprising the step ofapplying a base coat to the substrate before printing, the base coatcomprising the composition claimed in claim
 1. 19. The method claimed inclaim 18, wherein the substrate is foil, paper or board, or a plasticsmaterial.
 20. The method claimed in claim 19, wherein the plasticsmaterial is selected from polycarbonate, or poly(vinyl chloride).
 21. Amethod for varnishing a printed substrate, the method comprising he stepof applying a varnish to a printed substrate, characterized in that thevarnish comprises the composition claimed in claim
 1. 22. A substratehaving a base coat or varnish thereon, the base coat or varnishcomprising the composition of claim 1.